Earlier quoted context omitted.
The PS3 has an incredibly powerful CPU that is a a real pain to emulate. Modernretrogamer did quite an interesting Youtube piece on it: https://www.youtube.com/watch?v=lLebZyha74o
It's surprising to me that PS3 emulation (RPCS3) is so much further along than Xbox360 emulation (Xenia). Is it just a matter of one platform getting more attention?
Another factor is how the projects are maintained and managed. The maintainers of RPCS3 are more active and pull requests get feedback and merged relatively quickly.
On Xenia side, pull requests can remain open for months even though they should be merged, and this has caused multiple (potential) contributors to stop making pull requests.
Source: I'm a contributor on Xenia